Ni-Ti-Pd superelastic alloy material, its manufacturing method, and orthodontic archwire made of this alloy material

ABSTRACT

The present invention provides a Ni--Ti--Pd superelastic alloy material of a composition consisting of, by atomic percent, 34 to 49% nickel, 48 to 52% titanium and 3 to 14% palladium. Optionally, in a part of nickel and/or titanium of this alloy is replaced with one or more elements selected from a group of Cr, Fe, Co, V, Mn, B, Cu, Al, Nb, W and Zr such that these elements to be replaced amount to 2% or less in total (by atomic percent), wherein a stress hysteresis between the loading and unloading stresses in the stress-strain curve at temperatures between Af and Af+5° is as small as 50 to 150 MPa. Since the Ni--Ti--Pd superelastic alloy material having the above composition is excellent in hot workability, it can be hot-worked into a wire having a diameter up to the range from 1 to 5 mm and manufactured at a low cost. Then, a final heat-treatment is given to the hot-worked material at a temperature in the range from 300 to 700° C. through a step of final cold-drawing at a reduction ratio in a cross section area of not less than 20%, whereby an excellent superelastic material is obtained, with a stress hysteresis in the range from 50 to 150 MPa, and a residual strain of 0% or close to 0% after unloading, and which can be suitably used for an orthodontic archwire.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a Ni--Ti--Pd superelastic alloy material whichpresents a small stress hysteresis, its manufacturing method and anorthodontic archwire made of this alloy material.

2. Description of the Prior Art

In general, a metal material loaded with a stress exceeding its elasticlimit results in permanent deformation. However, a certain kind of alloysuch as Ni--Ti alloy has a function to go back to its original shapeafter unloading, even if a stress is loaded to provide such an alloywith a strain close to 10% at a temperature exceeding a reverseMartensite transformation finish (Af) temperature (which will behereinafter referred to as Af temperature) as shown in FIG. 1. Namely,the Ni--Ti alloy or the like has a superelastic function and is called asuperelastic alloy.

Incidentally, FIG. 1 is a typical graph of the stress-strain curve for asuperelastic alloy in loading and unloading. Referring to FIG. 1, 1represents a difference between a loading stress P₁ in the flat rangefrom a to b and an unloading stress P₂ in the flat range from c to d,and this difference is called a stress hysteresis.

The Ni--Ti alloy may be practically used for actuators, toys, pipecoupling or the like by taking advantage of its shape memory properties.In addition, the range of use of the Ni--Ti alloy by taking advantage ofits superelastic properties has been recently increasing. Thus, theNi--Ti alloy has been put into practical use in various fields by takingadvantage of stress-strain characteristics similar to rubber.Specifically, the Ni--Ti alloy is frequently used for frames of glasses,wires of brassieres, orthodontic archwires, antennas for portabletelephones or the like.

Further, an alloy having a composition adapted for such purposes hasbeen manufactured by adding a small amount of a metal element such asCr, Fe, Co, V, Mn and B to the Ni--Ti alloy in order to improve theworkability and alloy characteristics.

In general, a superelastic alloy material is deformed due to a moderatestress in loading and takes advantage of its high force in unloading.Thus, it is preferable that the superelastic alloy material goes back toits original shape due to a stress which is as close to the stress inloading as possible. Namely, the superelastic alloy material preferablypresents a small stress hysteresis. Further, it is necessary that aresidual strain should be 0%, or be close to 0% in unloading.

FIG. 2 is a graph of the stress-strain curve for a Ni--Ti superelasticalloy. As is apparent from FIG. 2, a stress hysteresis (shown by 1 inFIG. 2) is as large as approximately 300 to 400 MPa, and therefore,there has been a limit in the use of such a Ni--Ti superelastic alloy.

On the other hand, a Ni--Ti--Cu alloy has been developed as asuperelastic alloy which presents a small stress hysteresis. FIG. 3 is agraph of the stress-strain curve for the Ni--Ti--Cu superelastic alloy.

It has been found that the stress hysteresis of the Ni--Ti--Cu alloy isreduced as the amount of Cu in the alloy is increased, and that thestress hysteresis in a composition containing, by atomic percent,approximately 10% Cu is reduced down to 100 to 200 MPa and 20% Cu isreduced down to 40 MPa on a laboratory level (S. Miyazaki, I. Shiota, K.Otsuka and H. Tamura. Proc. of MRS Int'l. Mtg on Adv. Mats., Vol. 9(1989) Page 153, Hiroshi Horikawa and Tatsuhiko Ueki, Advanced Materials'93. V/B: Shape Memory Materials and Hydrides, edited by K. Otsuka etal. Trans. Mat. Res. Soc. Jpn., Volume 18B Page 1113, U.S. Pat. No.5,044,947). The Ni--Ti--Cu alloy having such a feature is mainly usedfor orthodontic archwires.

However, the hot workability of the Ni--Ti--Cu superelastic alloy isremarkably reduced as the amount of Cu in the alloy is increased. Thus,an alloy containing a large amount of Cu cannot be manufactured on afactory level. In addition, the stress hysteresis cannot be reduced toonly about 160 MPa under the existing circumstances.

Accordingly, the development of a superelastic alloy material which isexcellent in workability and presents an extremely small stresshysteresis has been needed. Incidentally, it is necessary for thesuperelastic alloy material that the residual stress in unloading asdescribed above should show 0% or be close to 0%.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide superelastic alloymaterials which present a small stress hysteresis by remedying theproblems in the above-mentioned prior art.

Another object of the present invention is to provide a method ofmanufacturing these alloy materials.

A further object of the present invention is to provide an orthodonticsuperelastic alloy wire which presents a small stress hysteresis.

The present invention for attaining the above objects has the followingfeatures.

Namely, in the first aspect of the present invention, there is provideda Ni--Ti--Cu superelastic alloy material which comprises a compositionconsisting of, by atomic percent, 34 to 49% nickel (Ni), 48 to 52%titanium (Ti), and 3 to 14% palladium (Pd), wherein a stress hysteresisbetween the loading and unloading stresses in the stress-strain curve attemperatures between Af and Af+5° is in the range from 50 to 150 MPa.

In the second aspect of the present invention, there is provided aNi--Ti--Pd superelastic alloy material, which is characterized in thatin the Ni--Ti--Pd alloy composition described above, a part of nickeland/or titanium is replaced with one or two or more elements selectedfrom a group consisting of Cr, Fe, Co, V, Mn, B, Cu, Al, Nb, W and Zrsuch that these elements to be replaced amount to 2% or less (by atomicpercent) in total, and a stress hysteresis between the loading andunloading stresses in the stress-strain curve at temperatures between Afand Af+5° is within the range from 50 to 150 MPa.

In the third aspect of the present invention, there is provided a methodof manufacturing a Ni--Ti--Pd superelastic alloy material, whichcomprises the steps of hot-working a slab of a Ni--Ti--Pd alloy havingthe composition as defined in any of the first and second aspects into awire having a diameter in the range from 1 to 5 mm, then repeatedlycold-drawing and annealing the hot-worked wire into a wire having apredetermined diameter at need, then annealing the wire having thepredetermined diameter, then cold-drawing the annealed wire into a wirehaving a final finish diameter at a reduction ratio in a cross sectionarea of not less than 20%, and giving a final heat-treatment to thecold-drawn wire at a temperature in the range from 300 to 700° C.,whereby a stress hysteresis between the loading and unloading stressesin the stress-strain curve at temperatures between Af and Af+5° is setto be in the range from 50 to 150 MPa.

Further, in the fourth aspect of the present invention, there isprovided an orthodontic archwire made of the Ni--Ti--Pd superelasticalloy material as defined in any of the first and second aspects of thepresent invention.

A detailed description will now be given of the present invention.

The Ni--Ti--Pd superelastic alloy material according to the presentinvention is provided as a superelastic alloy material which presents astress hysteresis smaller than that of the conventional Ni--Ti--Cu alloymaterial and is excellent in hot workability. A value of the stresshysteresis, i.e. difference between the loading and unloading stressesin the stress-strain curve for this alloy material at a temperatureexceeding the Af temperature, is in the range from 50 to 150 MPa. Thus,the stress hysteresis of the Ni--Ti--Pd alloy material of the presentinvention is extremely smaller than those of Ni--Ti and Ni--Ti--Cu alloymaterials, and its behavior in unloading is reverse of that in loading.

Incidentally, FIGS. 2, 3 and 4 show typical graphs of the stress-straincurves for Ni--Ti, Ni--Ti--Cu and Ni--Ti--Pd superelastic alloymaterials in loading and unloading at a temperature exceeding the Aftemperature, respectively.

In the Ni--Ti--Pd superelastic alloy material as the first aspect of thepresent invention, the amount of palladium (Pd) is set to be in therange from 3 to 14% (by atomic percent) for the following reasons.Namely, when the amount of palladium is less than 3%, an effect onreduction of the stress hysteresis will not be produced. On the otherhand, when the amount of palladium exceeds 14%, the cold workability isdegraded, and therefore, it is of no practical use.

In addition, the amount of nickel and that of titanium are set to be inthe range from 34 to 49% and in the range from 48 to 52% (by atomicpercent) for the following reasons. Namely, if the amount of nickel andthat of titanium are respectively outside the above range, theworkability is degraded and residual strain remains after unloading.

Accordingly, in the Ni--Ti--Pd superelastic alloy material of thepresent invention, the alloy composition consisting of, by atomicpercent, 50% titanium, 41 to 45% nickel and 5 to 9% palladium is themost preferable alloy composition, which will not only reduce the stresshysteresis but also obtain the satisfactory hot workability.

Further, in the superelastic alloy material as the second aspect of thepresent invention, a part of nickel and/or titanium in the Ni--Ti--Pdalloy composition described above is replaced with one or two or moreelements selected from a group consisting of Cr, Fe, Co, V, Mn, B, Cu,Al, Nb, W and Zr such that these elements to be replaced amount to 2% orless (by atomic percent) for the following reasons. Namely, suchreplacement makes it possible to improve the alloy characteristics suchas Af temperature and the workability of the alloy material, and to varythe alloy characteristics so as to be fit for the purpose.

According to the present invention in the first and second aspects, itis a matter of course that the stress hysteresis between the loading andunloading stresses in the stress-strain curve at temperatures between Afand Af+5° is set to be in the range from 50 to 150 MPa. The stresshysteresis as small as the range from 50 to 150 MPa obtained in thealloy material of the present invention is preferable from the viewpointof practical use. Incidentally, in the superelastic alloy material asdefined in the first and second aspects of the present invention, aresidual strain of not more than 0.5% is shown after unloading thestress which is loaded to provide such the superelastic alloy materialfor a strain up to 8%, as is apparent from examples 3 and 4 which willbe described later.

In the third aspect of the present invention, there is provided themethod of manufacturing the Ni--Ti--Pd superelastic alloy material. Inthe case of manufacturing this alloy material, the slab having the abovealloy composition is firstly hot-worked into a wire having a diameter inthe range from 1 to 5 mm. Since the Ni--Ti--Pd superelastic alloymaterial is excellent in hot workability, the slab is hot-worked intothe wire having the diameter as small as 1 to 5 mm as described above,and the cost of manufacture is sharply reduced.

Incidentally, with respect to the sectional shape of the wire having thediameter in the range from 1 to 5 mm, a wire having a circular sectionalshape is easily manufactured and is in common use. Alternately, a wiremay have an elliptical or square sectional shape. In this case, themaximum diameter of the elliptical or square wire should be in the rangefrom 1 to 5 mm as the wire diameter.

The wire hot-worked in this manner is repeatedly cold-drawn andannealed, as needed, to form a wire having a predetermined diameter,which is then annealed. Subsequently, the annealed wire having thepredetermined diameter is cold-drawn into a wire having a final finishdiameter at a reduction ratio in a cross section area of not less than20%. Further, a final heat-treatment is given to the cold-drawn wirehaving the final finish diameter at a temperature in the range from 300to 700° C.

Incidentally, although not restricted, it is preferable that the alloymaterial of the present invention is hot-worked at a temperature in therange from 700 to 900° C., and annealed at a temperature in the rangefrom 600 to 800° C.

The slab is hot-worked into the wire having the diameter in the rangefrom 1 to 5 mm as described above in order to do away with or reduce thesteps of cold-drawing and annealing along the manufacturing line as muchas possible. It is preferable that the slab is hot-worked into a wirehaving a diameter close to the final finish diameter.

As described above, for instance, in case a wire having the final finishdiameter of 1 mm is required and the diameter of the hot-worked wire is1.2 mm, the hot-worked wire having the diameter of 1.2 mm is cold-drawnto a diameter of 1 mm (a cold reduction ratio in a cross section area ofapproximately 30%) without the steps of cold-drawing and annealing alongthe manufacturing line. In another case the wire having the final finishdiameter of 1 mm is required and the diameter of the hot-worked wire is3.0 mm, the hot-worked wire having the diameter of 3.0 mm is repeatedlycold-drawn and annealed to form a wire having a diameter of 1.2 mm,which is then annealed. Subsequently, the annealed wire is cold-drawninto a wire having a diameter to 1 mm (a final cold reduction ratio in across section area of approximately 30%).

The wire cold-drawn at the reduction ratio in a cross section area ofnot less than 20% as described above is finally heat-treated at atemperature in the range from 300 to 700° C. The reason why the wire iscold-drawn at the above reduction ratio and then heat-treated in thismanner is that satisfactory superelastic properties are obtained in theabove range of the reduction ratio and that of the temperature for heattreatment without any residual strain after unloading.

Incidentally, the final sectional shape of the wire may be circular,elliptical or square. Further, although the reduction ratio in a crosssection area of not less than 20% is required in the cold-drawing, itsupper limit is approximately 60%. When the reduction ratio exceeds 60%,there is a fear of the occurrence of breakage.

In the fourth aspect of the present invention, there is provided theorthodontic archwire made of the Ni--Ti--Pd superelastic alloy materialhaving the alloy composition as defined in any of the first and secondaspects of the present invention, wherein the stress hysteresis betweenthe loading and unloading stresses in the stress-strain curve attemperatures between Af and Af+5° is in the range from 50 to 150 MPa.

When the superelastic alloy material is used for the orthodonticarchwire, it is desired that a certain amount of tensile force should beapplied in working (loading) to attach the wire to the teeth, and that atensile force should be as high as possible in unloading to move theteeth after attaching the wire to the teeth, namely, the stresshysteresis should be small. Thus, it can be said that the Ni--Ti--Pdsuperelastic alloy material of the present invention hascharacteristics, which have not been found until now, suitable for anorthodontic archwire, since its stress hysteresis is as small as 50 to150 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and features of the invention willbecome apparent from the following description of preferred embodimentsof the invention with reference to the accompanying drawings, in which:

FIG. 1 is a typical graph showing the stress-strain curve for asuperelastic alloy in loading and unloading, in which 1 represents astress hysteresis;

FIG. 2 is a graph showing the stress-strain curve for a Ni--Tisuperelastic alloy in loading and unloading, in which 1 represents astress hysteresis;

FIG. 3 is a graph showing the stress-strain curve for a Ni--Ti--Cusuperelastic alloy (Ni--Ti--Cu--Cr alloy) in loading and unloading, inwhich 1 represents a stress hysteresis; and

FIG. 4 is a graph showing the stress-strain curve for a Ni--Ti--Pdsuperelastic alloy according to the present invention in loading andunloading, in which 1 represents a stress hysteresis.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

Superelastic alloy materials having the composition shown in Table 1were manufactured on an experimental basis as an example of the presentinvention and a comparative example. Namely, each alloy compositionshown in Table 1 was melted and cast into a slab, which was thenhot-rolled at a temperature in the range from 750 to 850° C. into a wirehaving a diameter of 3 mm. Then, this hot-rolled wire was repeatedlycold-drawn and annealed into a wire having a predetermined smalldiameter (about 1.2 mm), which was then annealed (at a temperature of700° C.). The annealed wire was cold-drawn at a reduction ratio in across section area of 30% into a wire having a diameter of 1 mm.Thereafter, a final heat-treatment was given to this cold-drawn wire for60 minutes at a temperature of 400° C. to manufacture a test material.Incidentally, in the hot working, Ni--Ti--Pd alloy materials (Nos. 1 to10 in Table 1) were excellent without any cracks the surface of thewire. On the other hand, Ni--Ti--Cu alloy materials (Nos. 11 and 12 inTable 1) caused cracks on the surface of the wire. Thus, with respect tothe Ni--Ti--Cu alloy materials, the satisfactory portions of thehot-worked material were used for the following working of the wire, anda test material having a diameter of 1 mm was manufactured. Thestress-strain curves for these wires in loading and unloading wereobtained.

A test was made to unload a stress which had been loaded so as toprovide the test material with a strain up to 4% with the testtemperature was set to be in the range of temperatures between Af andAf+5° of each alloy. The difference (stress hysteresis) between loadingand unloading stresses was found from the stress-strain curve, and theresults obtained are shown in Table 1.

In addition, Af temperature (reverse Martensite transformation finishtemperature) of each alloy was measured by means of thermal analysis,and the results obtained are shown in Table 1. The unloading stresses(MPa) under the strain of 2% are also shown in Table 1 for reference.

                                      TABLE 1    __________________________________________________________________________                                  Unloading             Alloy    Af          stress under                                        Stress             composition                      temperature                            Hot   strain of 2%                                        hysteresis    No       (at %)   (°C.)                            workability                                  (MPa) (MPa)    __________________________________________________________________________    Example of          1  Ni.sub.46.5 Ti.sub.49.5 Pd.sub.4                       5    ∘                                  240   137    present          2  Ni.sub.42.5 Ti.sub.50 Pd.sub.7.5                      25    ∘                                  340    95    invention          3  Ni.sub.40.5 Ti.sub.49.5 Pd.sub.10                      -20   ∘                                  340    54          4  Ni.sub.35 Ti.sub.51 Pd.sub.14                      60    ∘                                  170   127          5  Ni.sub.47 Ti.sub.49.5 Pd.sub.3 Cr.sub.0.5                       5    ∘                                  220   148          6  Ni.sub.36 Ti.sub.49 Pd.sub.13 Fe.sub.2                      -30   ∘                                  200   103          7  Ni.sub.42 Ti.sub.50 Pd.sub.7.5 Co.sub.0.5                      15    ∘                                  350    82          8  Ni.sub.38 Ti.sub.49.5 Pd.sub.12 V.sub.0.5                      30    ∘                                  180   106    Comparative          9  Ni.sub.47.5 Ti.sub.50 Pd.sub.2.5                      55    ∘                                  205   263    Example          10 Ni.sub.36 Ti.sub.49 P.sub.15                      -50   ∘                                  165   170          11 Ni.sub.43.5 Ti.sub.49.5 Cu.sub.7                      25    x     380   236          12 Ni.sub.40 Ti.sub.50 Cu.sub.10                      60    x     395   172    __________________________________________________________________________     Note) In Table, ∘ represents that no crack is produced on the     surface of the wire in hot working. x represents that the surface of the     wire is cracked in hot working.

As is apparent from Table 1, with respect to the Ni--Ti--Cu alloys ofNos. 11 and 12, cracks were produced in hot working, and the yield wasremarkably reduced. The Ni--Ti--Cu alloy of No. 12 presented a stresshysteresis of 172 MPa and showed relatively satisfactorycharacteristics. However, considering the hot workability, it isdifficult to manufacture the Ni--Ti--Cu alloy on a factory level, andfurther reduction of the stress hysteresis is not expected in theNi--Ti--Cu alloy.

On the other hand, the Ni--Ti--Pd alloys of Nos. 1 to 10 were excellentin hot workability. Namely, each of these Ni--Ti--Pd alloys could behot-worked into a wire having a diameter to 3 mm without producing anycracks on the surface of the wire in hot rolling, differently from theNi--Ti--Cu alloy materials. The Ni--Ti--Pd alloys of Nos. 9 and 10, thecomparative examples, were excellent in workability, but presented alarge stress hysteresis. Thus, as a result, the Ni--Ti--Pd alloys ofNos. 9 and 10 are not superior in characteristics to the conventionalNi--Ti--Cu alloys. On the other hand, the Ni--Ti--Pd alloys of Nos. 1 to8 as the example of the present invention were excellent in hotworkability, and the stress hysteresis was as small as 50 to 150 MPa,which were not only reduced down to 1/2 to 9/10 in comparison with thestress hysteresis of the Ni--Ti--Cu alloys, but also exhibited excellentformability in hot-working.

Further, the alloys of Nos. 5 to 8 were prepared by replacing a part ofnickel and titanium in the Ni--Ti--Pd alloy with the elements Cr, Fe, Coand V, respectively. When the amount of Cr, Fe, Co or V exceeds 2% (byatomic percent) in total, the workability was remarkably reduced, andtherefore, its working was impossible. Further, as long as the elementsto be replaced amount to 2% or less (by atomic percent) in total,although two or more elements are added in the alloy, it is possible tomanufacture a Ni--Ti--Pd superelastic alloy which is excellent in hotworkability and presents a stress hysteresis as small as 80 to 150 MPa.

Example 2

Table 2 shows an example of the present invention and a comparativeexample in case of varying the temperature for final heat-treatmentafter cold-working.

Specifically, the alloy of No. 2 shown in Table 1 was cold-drawn at areduction ratio in a cross section area of 30% into a wire having thediameter of 1 mm on an experimental basis, similarly to the example 1.Subsequently, a final heat-treatment was given to this cold-drawn wireon conditions shown in Table 2.

The wire thus manufactured was tested similarly to the example 1, exceptthat a stress was loaded to provide this wire with a strain up to 8%.Then, the stress-strain curves in loading and unloading were obtained.The residual strain (%) after unloading and the stress hysteresis (MPa)in this case were found, and the results obtained are shown in Table 2.

                  TABLE 2    ______________________________________           Heat-treatment    Residual  Stress           temperature                    Time     strain    hysteresis           (°C.)                    (min.)   (%)       (MPa)    ______________________________________    Comparative             250        60       Breakage                                         --    example    Example  300        30       0.2     83    of       450        60       0.0     95    present  700        5        0.1     93    invention    Comparative             750        5        1.5     90    example    ______________________________________

As is apparent from Table 2, the material heat-treated at thetemperature of 250° C. was broken under the strain of 7%. In addition,the material heat-treated at the temperature of 750° C. is not suitablefor a super-elastic alloy material, since the high residual strain afterunloading is found. On the other hand, it can be seen that the materialsheat-treated at temperature in the range from 300 to 700° C. result inexcellent superelastic alloy materials, since the residual strain isapproximately 0% and the stress hysteresis is as small as 80 to 100 MPa.

Example 3

The alloy having the composition shown in No. 3 of Table 1 washot-worked into a wire having a diameter to 3 mm, similarly to theexample 1. The wire thus hot-worked was then repeatedly cold-drawn andannealed into a wire having a predetermined small diameter, which wasthen annealed (at the temperature of 700° C.). Then, the annealed wirewas drawn at a reduction ratio in a cross section area shown in Table 3to manufacture a wire having a diameter of 1 mm on an experimentalbasis. Thereafter, a final heat-treatment was given to this wire for 60minutes at the temperature of 400 ° C.

The wire thus manufactured was tested similarly to the example 1, exceptthat a stress was loaded to provide this wire with a strain up to 8%,and the stress-strain curves in loading and unloading were obtained. Theresidual strain (%) after unloading and the stress hysteresis (MPa) inthis case were found, and the results obtained are also shown in Table3.

                  TABLE 3    ______________________________________            Reduction ratio                         Residual Stress            in cross section area                         strain   hysteresis            (%)          (%)      (MPa    ______________________________________    Comparative              0              2.0      113    example   10             1.5      104    Example   20             0.3      93    of        30             0.0      95    present   50             0.2      85    invention    ______________________________________

As is apparent from Table 3, with respect to all samples worked at thereduction ratio in a cross section area of 0% and 10%, high residualstrain is found. On the other hand, with respect to those samples workedat the reduction ratio in a cross section area of not less than 20%, theresidual strain is as small as approximately 0%. Accordingly, it ispossible to provide an excellent superelastic alloy, in which theresidual strain is approximately 0%, and the stress hysteresis is in therange from approximately 80 to 100 MPa at the reduction ratio in a crosssection area of not less than 20%.

Example 4

The following test was made on the assumption that the alloy material ofthe present invention is used as an orthodontic archwire.

A slab having an alloy composition shown in Table 4 was hot-worked intoa wire having a diameter of 3 mm. Then, this hot-worked wire wasrepeatedly cold-drawn and annealed into a wire having a diameter of 0.56mm, which was then annealed at the temperature of 700° C. Then, theannealed wire was finally cold-drawn at the reduction ratio in a crosssection area of 35% into a wire having a diameter of 0.45 mm.Thereafter, a final heat treatment was given to the finally cold-drawnwire under the conditions shown in Table 4 to provide an orthodonticarchwire.

The orthodontic archwire thus manufactured was tested to obtain thestress-strain curves for loading and unloading. The stress hysteresis(MPa) was obtained by unloading a stress which has been loaded toprovide this archwire with a stress up to 8%, with the test temperatureset to 30° C.

The alloy composition, the temperature for heat treatment and the stresshysteresis in this test are shown in Table 4.

                                      TABLE 4    __________________________________________________________________________    Alloy          Af         Unloading stress                                       Stress    composition    temperature                         Heat under strain of 2%                                       hysteresis    (at %)         (°C.)                         treatment                              (MPa)    (MPa)    __________________________________________________________________________    Example of          Ni.sub.42.5 Ti.sub.50 Pd.sub.7.5                   30    500° C.                              190      130    present    invention    Comparative          Ni.sub.44.5 Ti.sub.50 Cu.sub.5.0 Cr.sub.0.5                   30    400° C.                              200      175    example    __________________________________________________________________________

With respect to alloys of the examples of both the present invention andthe comparative examples, the residual strain after unloading was assmall as 0.2% or less, and satisfactory superelastic characteristicswere obtained. In addition, while the stress hysteresis of a Ni₄₄.5 Ti₅₀Cu₅.0 Cr₀.5 alloy material as the comparative example was 175 MPa, thestress hysteresis of a Ni₄₂.5 Ti₅₀ Pd₇.5 alloy material as the exampleof the present invention was 130 MPa, which was approximately threequarters that of the comparative example.

As is apparent from the results of this test, in the case of thesuperelastic alloy material of the present invention used as aorthodontic archwire, the following effects are obtained. Namely, incase of attaching the wire to the teeth, the wire can be fastened to theteeth by a tensile force which is as small as possible. In addition, theteeth can be moved by a force which is as high as possible.

As described in the foregoing, the present invention provides theNi--Ti--Pd superelastic alloy material, which comprises a compositionconsisting of, by atomic percent, 34 to 49% nickel, 48 to 52% titaniumand 3 to 14% palladium, or the Ni--Ti--Pd superelastic alloy material,which is characterized in that a part of nickel and/or titanium of thisalloy is replaced with one or more elements selected from a groupconsisting of Cr, Fe, Co, V, Mn, B, Cu, Al, Nb, W and Zr such that theelements to be replaced amount to 2% or less (by atomic percent) intotal, wherein a stress hysteresis is as small as 50 to 150 MPa. Sincethe Ni--Ti--Pd superelastic alloy material having the above compositionis excellent in hot workability, it can be hot-worked into a wire havinga small diameter up to the range from 1 to 5 mm, and also can bemanufactured at a low cost. Then, the final cold-drawing is given to thehot-worked wire at the reduction ratio in a cross section area of notless than 20%, and thereafter, the final heat-treatment is given to thefinally cold-drawn wire at a temperature in the range from 300 to 700°C., whereby an excellent superelastic alloy material is provided, whichpresents a stress hysteresis in the range from 50 to 150 MPa and aresidual strain of 0% or close to 0% after unloading.

Further, when the superelastic alloy material of the present inventionis used as an orthodontic archwire, excellent characteristics can beobtained in attaching the wire to the teeth and in treatment.

What is claimed is:
 1. A Ni--Ti--Pd superelastic alloy compositionconsisting of, by atomic percent, 34 to 49% nickel (Ni), 48 to 52%titanium (Ti) and 3 to 14% palladium (Pd);wherein said alloy has astress hysteresis, between the loading and unloading stresses in thestress-strain curve at temperatures between Af and Af+5°, in the rangefrom 50 to 150 MPa and a residual strain of 0.5% or less.
 2. ANi--Ti--Pd superelastic alloy composition consisting of, by atomicpercent of the alloy composition, 34 to 49% Ni, 48 to 52% Ti, 3 to 14%Pd and 0 to 2% of one or more elements selected from the groupconsisting of Cr, Fe, Co, V, Mn, B, Cu, Al, Nb, W and Zr replacing apart of said Ni and/or said Ti, said alloy composition having a stresshysteresis difference, between the loading and unloading stresses instress-strain curves at temperatures between Af and Af+5°, in a rangefrom 50 to 150 MPa and a residual strain of 0.5% or less.
 3. A method ofmanufacturing a Ni--Ti--Pd superelastic alloy material, comprising thesteps of:hot-working a slab having an alloy composition according toclaim 1 into a wire having a diameter in the range from 1 to 5 mm; thenrepeatedly cold-drawing and annealing the hot-worked wire to form a wirehaving a predetermined diameter; then annealing the wire having thepredetermined diameter; subsequently cold-drawing the annealed wire intoa wire having a final finish diameter at a reduction ratio, incross-sectional area, of not less than 20%; then heat-treating thecold-drawn wire at a temperature in the range from 300 to 700° C. toimpart the wire with a stress hysteresis, between the loading andunloading stresses in the stress-strain curve at temperatures between Afand Af+5°, in a range from 50 to 150 MPa and a residual strain of 0.5%or less.
 4. An orthodontic archwire, comprising:the Ni--Ti--Pdsuperelastic alloy composition according to claim
 1. 5. An orthodonticarchwire, comprising:the Ni--Ti--Pd superelastic alloy compositionaccording to claim
 2. 6. An orthodontic archwire according to claim 4produced by a method comprising the steps of:hot-working a slab havingsaid alloy composition into a wire having a diameter in the range from 1to 5 mm; then repeatedly cold-drawing and annealing the hot-worked wireto form a wire having a predetermined diameter; then annealing the wirehaving the predetermined diameter; subsequently cold-drawing theannealed wire into a wire having a final finish diameter at a reductionratio, in cross-sectional area, of not less than 20%; then heat-treatingthe cold-drawn wire at a temperature in the range from 300 to 700° C. toproduce a wire with a stress hysteresis, between the loading andunloading stresses in the stress-strain curve at temperatures between Afand Af+5°, in a range from 50 to 150 MPa.
 7. An orthodontic archwireaccording to claim 5 produced by a method comprising the stepsof:hot-working a slab having said alloy composition into a wire having adiameter in the range from 1 to 5 mm; then repeatedly cold-drawing andannealing the hot-worked wire to form a wire having a predetermineddiameter; then annealing the wire having the predetermined diameter;subsequently cold-drawing the annealed wire into a wire having a finalfinish diameter at a reduction ratio, in cross-sectional area, of notless than 20%; then heat-treating the cold-drawn wire at a temperaturein the range from 300 to 700° C. to produce a wire with a stresshysteresis, between the loading and unloading stresses in thestress-strain curve at temperatures between Af and Af+5°, in a rangefrom 50 to 150 MPa.